For years, Firefox has trundled along at the back of the browser pack, a beast of burden laden with fancy features but lacking speed. Not anymore. Mozilla has released Firefox 4, and in our preliminary tests, the browser makes a huge performance leap forward.

On a 2GHz Core 2 Duo aluminum MacBook with 2GB of RAM, Firefox 4 roughly quadrupled Firefox 3.6.15's speed in an XHTML rendering test and the SunSpider JavaScript benchmark. Its Acid3 score improved by 3 points (reaching 97 out of 100), and it gained more than 100 points out of 400 on its predecessor in an HTML5 compliance test (255 and 9 bonus points vs. 155 and 4 bonus points).

Against other browsers, Firefox 4's XHTML and CSS rendering speed still trails the latest versions of Safari, Opera, and Chrome, and it lacks their perfect Acid3 scores. But its HTML5 compliance outscores Safari's and Opera's, and only slightly lags Chrome's, and in the SunSpider test, Firefox 4's JägerMonkey JavaScript engine beats everyone else.

Firefox 4 packs a few other happy improvements under the hood. It now offers full support for CSS3 transforms and transitions, and hugely enhanced support for CSS3 animations. Built-in hardware acceleration vastly speeds up page rendering; however, in a Mozilla-created "stress test," my computer and browser only eked out 2 frames per second when attempting 3D transformations on multiple 2D images. Firefox 4's superb support for WebGL 3D graphics, Google's new WebM open source video codec, and OpenType font rendering left me much more impressed.

Firefox 4's interface changes are a somewhat mixed bag. Tabs now top the URL bar, Chrome-style, and you can now jump directly to a tab by starting to type its name in the "awesome bar" --nice, but nothing major. I liked the "app tab" feature, which lets you shift Gmail, Google Docs, and other much-used Web apps into tiny, space-saving icon tabs at the far left of the tab list.

The ability to group tabs by dragging and dropping left me cold, though. You must switch to an entirely separate screen, and drag tabs one by one into groups. The whole process just added an extra layer of clutter and confusion to my browsing.

We're working on a full review of Firefox 4. For now, Firefox 4 looks like a happy compromise between speed-demon performance and a polished interface.